Anti-lock braking systems (ABS) are increasingly popular on present day motor vehicles. Such systems automatically prevent wheel lock-up during hard braking maneuvers by modulating braking pressure in such conditions so that vehicle stability and directional control can be maintained. A critical feature of an ABS is a wheel speed sensor which provides a signal to the ABS controller related to wheel rotation. Passenger cars equipped with ABS typically have such a sensor for each of their four wheels. By monitoring the sensor signals, the braking system controller can determine if a wheel lock-up condition is being approached and thus control the braking system. Wheel system sensors are also used to provide signals for traction control systems which reduce slippage during acceleration. Traction control systems enhance traction by selectively applying braking torque to a spinning wheel causing torque transfer to a non-spinning traction wheel.
Numerous designs of ABS wheel speed sensors are presently known. Such sensors generally consist of a rotating part in close proximity to a stationary part. The rotating part, or "tone ring" has features which can be sensed as they pass the stationary part. Such features are typically ferromagnetic teeth, as on a gear, or magnetic poles which have been applied to the rotating part. The stationary part includes a transducer which can detect the passing of the features as the tone ring rotates. The detection is indicated by an electrical signal which is emitted by the transducer. The transducer may be a variable reluctance device, Hall effect device, magneto-resistive device, or of some other construction. However, some of these systems have problems sensing low/zero speed conditions. Light systems using rotating shutters are also known but do not work reliably in hostile automotive environments.
The use of optical sensors for determining the speed and/or torque of a rotating member has also been contemplated. Automobile manufacturers will eventually switch to optical fiber data communication systems to increase data capacity while reducing weight and cost. Optical sensors typically rely on modulating or changing a characteristic of electromagnetic energy and use a photoelectric device to convert that modulated characteristic into an electrical signal that can be further processed with conventional electronics. For example, optical speed sensors are known that utilize the Faraday rotation effect to detect the rotational speed of an object. Such an apparatus is shown in U.S. Pat. No. 4,947,035 issued to Zook et al. The "Domain diffraction" effect has also been used to detect the rotational speed of an object as disclosed in U.S. Pat. No. 5, 192,862 issued to Rudd, Ill.
One disadvantage of the Faraday effect system is that it requires a polarization filter, which adds to the cost of the system. Another disadvantage is that the Faraday effect system requires a multi-mode fiber.
A potential feature of "Domain diffraction" effect systems when used with a multipole magnet frequency doubling. Frequency doubling occurs because the sensor is sensing magnetic strength, and not magnetic direction. Thus, the optical signal reaches a low point when the magnetic signal crosses zero, and the optical signal reaches a high point when the magnetic signal is at a positive or negative peak. If the magnetic fields have some bias ("imbalance" or "offset", where positive fields are not the same strength as negative fields), then the output cycles corresponding to the weaker pole direction will be smaller than those corresponding to the stronger poles. If the bias is very strong, there may be no flux reversal at all. In this case, the output frequency of the sensor would be the same as the frequency of the magnetic excitation (frequency doubling is completely lost).
Thus there is a need for an improved fiber optic sensor system for detecting movement or position of a rotating wheel bearing for use in anti-lock braking systems and traction control systems. There is also a need for reducing the cost needed to implement fiber optic sensor systems in anti-lock braking systems and traction control systems. There is yet a further need for an improved fiber Optic sensor system which is in a completely encased and sealed environment for packaging within a bearing assembly.